Theoretical study of magnetic odering of defects in diamond

Abstract:

Magnetic ordering of dopants in diamond holds the prospect of exploiting diamond’s unique
properties in the emerging field of spintronics. Several transition metal defects have been
reported to order ferromagnetically in various semiconductors, however, low Curie
temperatures and lack of other fundamental material properties have hindered practical
implementation in room temperature spintronic applications. In this Thesis, we consider the
energetic stability of 3d transition metal doped-diamond and its magnetic ordering properties
at various lattice sites and charge states using ab initio Density Functional Theory methods.
We find the majority of 3d transition metal impurities in diamond at any charge state to be
energetically most stable at the divacancy site compared to substitutional or interstitial lattice
sites, with the interstitial site being highly unstable (by ~8 - 10 eV compared to the divacancy
site). At each lattice site and charge state, we find the formation energies of transition metals
in the middle of the 3d series (Cr, Mn, Fe, Co, Ni) to be considerably lower compared to
those early or late in the series. The energetic stability of transition metal impurities across
the 3d series is shown to be strongly dependent on the position of the Fermi level in the
diamond band gap, with the formation energies at any lattice site being lower in p-type or ntype
diamond compared to intrinsic diamond.
Further, we show that incorporation of isolated transition metal impurities into diamond
introduces spin polarised impurity bands into the diamond band gap, while maintaining its
semiconducting nature, with band gaps in both the spin-up and spin-down channels. These
impurity bands are shown to originate mainly from s, p-d hybridization between carbon sp
3
orbitals with the 3d orbitals of the transition metal. In addition, the 4p orbitals contribute
significantly to hybridization for transition metal atoms at the substitutional site, but not at
the divacancy site. In both cases, the spin polarisation and magnetic stabilization energies are
critically dependent on the lattice site and charge state of the transition metal impurity.
By allowing magnetic interactions between transition metal atoms, we find that ferromagnetic
ordering is likely to be achieved in divacancy Cr+2, Mn+2, Mn+1 and Co0 as well as in
substitutional Fe+2 and Fe+1, indicating that transition metal-doped diamond is likely to form
a diluted magnetic semiconductor which may successfully be considered for room
temperature spintronic applications. In addition, these charge states correspond to p-type
diamond, except for divacancy Co0, suggesting that co-doping with shallow acceptors such as
B (
will result in an increase of charge concentration, which is likely to
enhance mediation of ferromagnetic spin coupling. The highest magnetic stabilization energy
occurs in substitutional Fe+1 (33.3 meV), which, also exhibits half metallic ferromagnetic
ordering at the Fermi level, with an induced magnetic moment of 1.0 μB per ion, thus
suggesting that 100 % spin polarisation may be achieved in Fe-doped diamond.

Citation:Benecha, Evans Moseti (2011) Theoretical study of magnetic odering of defects in diamond, University of South Africa, Pretoria, <http://hdl.handle.net/10500/6312>

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